Fix: TDA2822 Audio Amp (v3 boards only)

Introduction

The TDA2822 is the audio amplifier part located on the v3 main board of the µBITx. The BITX40 and earlier models used the LM386 as an audio amplifier, so this board was new to the µBITx. The part has been replace in the v4 board currently shipping by discrete 2N3904/6 transistors.

Two problems have emerged with the TDA2822 part:

some parts are failing spontaneously (this seems to be limited to the WX branded parts incorporated late in Batch 2 and early in Batch 3); and

Some have managed to blow up their TDA2822 (regardless of brand of the part). This seems to relate to shorted outputs, normally as a result of inserting a mono plug into the stereo jack OR by turning up the audio volume control to a high level.

Supply Problems and Variable Component Quality

Ashhar Farhan VU2ESE, designer of the µBITx tells us the sorry saga of the TDA2822:

“Here is the story. This is going to cause a lot of heartaches. I chose TDA2822 after listening to a lot of bad press about the LM386. Upon looking at its harmonic distortions et al, it was found to be a reasonably good device. In, it went.

“Now, unfortunately, the TDA2822 production has entirely stopped. None of the standard suppliers, including mouser, newark, element14 have stocks left. We tried to find another source of remaining stock from a reliable supplier who has been supplying us in the past.

“He turned up with a batch. We tried the ICs in burn tests: ten of them on ten boards at full volume for a whole day. In retrospect, we should have tested every individual IC. That batch had these WX ICs as well. About 100 to 150 of these must have shipped, there is no way to know how many.

“The way it works is like this : Not all the ICs turned out by a factory are good. Some work very well, some not so well, some don’t work at all. Those that don’t work so well make it to the silicon industry underground where they are cut out from the wafers and packaged and sold as low grade versions of the same parts. We got a a hundred or so of these lemons.

“We finally located a source of brand new, high quality TDA2822s that are currently being shipped. They cost almost five times as much as we were paying while this was in production. Well, such is life.

“In the meantime, we have to look for alternatives to the TDA2822. I am highly inclined towards making a discrete device amplifier.”

So how do I know whether I have a bad part?

If you have a WX marked part then you probably need to replace the part or supply the part with a lower voltage via a regulator (see below for further details) in order to avoid spontaneous failure.

If you have an FCI marked part then you should be less concerned. These tend to fail when the output is shorted, such as plugging a mono plug into a stereo jack.

Modes of failure

There seem to be three potential failure modes:

A known run of bad chips (WX type) that seem to “let out the smoke” spontaneously. It is thought that these are probably low voltage versions that have been incorrectly labelled or shop floor sweepings.

Loud pops or extra loud signals may be caused by the rapid current in rush in charging up or discharging the 470µF capacitor (C77) in the output circuit, thus damaging the chip.

High voltage causing the device to exceed specified dissipation limits. The circuit is running near its voltage maximum (rated for 15v, but most circuit designs are intended for much lower voltages).

ubitx.net Advice for v3 Board OWNERS

If you have the WX chip, you need to either replace it or reduce the voltage supplied to the chip. If you don’t replace the chip, or reduce the voltage supplied to the chip, it will almost certainly fail in service.

If you have the FCI chip or another brand, then you may, at least, want to consider one of the three fixes below that would provide some protection for your TDA2822. If you regularly insert plugs into the stereo phone jack or are hard of hearing, then you would be advised to complete one of these mods.

Replacing your TDA2822

If you need to replace the TDA2822, it should not be difficult to find a replacement IC. If you don’t mind waiting these can be procured very cheaply from the Far East. If you want it more quickly, then they can be order through a local supplier. It is worth investing more in a name brand replacement (such as the FCI version or a part from a major supplier). Also consider the NJM2073D as it is a pin for pin replacement.

Make sure that you also purchase an 8 pin DIL socket (or use machine pin headers). This will make it easier to replace the chip in future if it blows again.

Removal of the IC is easily achieved by snipping the pins above the board to remove the chip, and then removing the soldered pins one by one using a soldering iron applied to the pin underneath the board with some solder wick and use needle nose pliers to leverage out the pin from the top side of the board. Wedge the board vertical when you do this in a vice or just propped up on your workbench. Take care you don’t damage the board when removing the device.

Replacing your TDA2822 with an LM386 Module

You could use an LM386 module as a replacement to the audio amplifier on the µBITx board. LM386 modules are readily available on the internet for well under a US$1. Buy several so you have spares in the junk box. Source the audio feed from the volume control output and wire up the stereo jack or speaker to the output of the module.

Replacing your TDA2822 with an LM386 using a DIL socket adapter

The TDA2822 and LM386 are not pin compatible, but it is possible to make an adapter to plug into the TDA2822 DIL socket (using two further DIL sockets).

The pinout map supplied byClaude HB9CGL is as follows:

LM386 pin – TDA2822 corresponding pin

2 – 4
3 – 7
4 – 4
5 – 1
6 – 2

Howard K4LXY shows his “adapter” using the LM386 to replace the TDA2822:

Claude has left pins 1 and 8 of the LM386 unattached, and in this configuration the LM386 datasheet states that it has a voltage gain of 20, or 20*log10(20) = 26dB. The LM386 on the Bitx40 has a 0.1uF cap between pins 1 and 8 to increase the gain to 46dB. The original TDA2822 of the uBitx has a gain of 40dB, so you may want to try adding a cap between LM386 pins 1 and 8 if you need more gain there. A larger value cap (up to 10uF) would improve low frequency response. Howard decided to include this capacitor.

Fixes to reduce the risk of damage occurring to your TDA2822

Adding a resistor in series with the 12V line maybe 10-15 ohms to drop the voltage.

Using a 16 Ohm speaker or add a series 8 to 10 Ohm resistor with 8 ohm speakers.

Cutting the power track to the TDA and insert a regulator (e.g. 78L09) to reduce the voltage (this could be a DIL or SMD type). There is a convenient track that can be cut. Solder the regulator onto the board at this point. The track leads directly to the chip and filter cap (see photo below).

The center lead looks soldered, but is in fact floating. However, the tab is soldered to the copper ground-plane below. The white wire is part of Raj’s fix to prevent the filter relays from clicking during PTT. Raj suggests the board as illustrated now needs a brush cleaning!

Raj observes that with this mod, audio is fine but distorts at high volume. The 9V voltage out does not drop at maximum volume.

Philip G7JUR, who has completed the the regulator mod suggests that if you are using a 4 ohm speaker, then use a 7806, whereas if you are using an 8 ohm speaker then use a 7809. 6 volts for a 4 ohm speaker give a max of 650 mW, and 9 volts for a 8 ohm speaker gives a max of 1000 mW.